Such a method is known from US 2005/0015200 A1. The known method is used to transmit satellite data from the satellites to the navigation device. The satellite data contain the information needed for a navigation device to determine its own position. The information is distributed in so-called navigation messages. The navigations messages contain almanacs with information on the clock and orbit of the other satellites. The known method includes steps for transmitting almanac data from a base station to a navigation device by a wireless connection between the base station and the navigation device. The base station receives a complete set of almanacs from a data source in the internet or from a receiver for the global positioning system. The base station then creates a subset of the complete set of satellite almanacs. The selection of the subset depends on the position of the navigation device. Finally, the subset of the current satellite almanac is transmitted from the server to the wireless device. According to the known method, a subset of the complete set of almanacs comprises the almanac data of the satellites, which are not in view of the navigation device. Additionally the more accurate ephemeris data for one or more satellites that are in view might be transmitted.
An almanac should be understood as a set of clock and orbital data of reduced precision of a single satellite. According to NAVSTAR, “GPS Standard Positioning Service: Signal Specification”, available online: www.navcen.uscg.gov/pubs/gps/sigspec, 2nd ed., June 1995, the almanac data are contained in the 25 pages of the navigation message. The almanac data for the i-th satellite are implemented in the fourth and fifth subframe of the i-th page. The transmission of one page takes 30 seconds resulting in a total transmission time of 12.5 minutes for the fundamental Global Positioning System (=GPS) constellation with 24 satellites.
One purpose of the almanac is to initialize signal acquisition when a new satellite rises above the horizon and the user position is known. The second purpose of the almanac is to accelerate the signal acquisition of a warm start. The warm start is an acquisition mode that is based on a priori information of the last user position, the receiver clock time and the complete set of almanacs such that the search of the code delay Δτ and the Doppler shift Δfd of a specific satellite is significantly simplified.
A cold start is defined by a signal acquisition without any a priori information. The signal acquisition of the first satellite is generally very time consuming as the search space (Δτ, Δfd) is very large. After acquisition and carrier tracking, the navigation message is demodulated and the almanac is read. This information is of fundamental importance for the signal acquisition of further satellites.
The transmission of a complete set of almanacs by all satellites has the disadvantage that the time of a cold start can be quiet long. Therefore, the known method disclosed in US 2005/0015200 A1 reduces the redundancy of the almanac broadcast by selecting a subset of the almanac data that is actually needed by the navigation device. However, the known method presumes a wireless connection between the navigation device and the base station. This implies that the navigation device is provided with a receiver for a mobile network. Finally, the base station must send specific messages to the navigation devices depending on its position, which increases the system load of the mobile network.
U.S. Pat. No. 6,671,620 B1 discloses a further method for providing almanac data to a navigation device. In the known method the navigation device receives its rough position from a base station. The navigation device determines a list of visible satellites based on the rough position and the available almanac data. Finally, the navigation device transmits a list of visible satellites to the base station and requests the missing almanac data from the base station.
U.S. Pat. No. 4,445,118 proposes a navigation system based on a constellation of orbiting satellites. In the navigation system an acquisition-aiding signal generated by an earth-based control station is relayed to a navigation device via a geostationary satellite. The aiding signal contains almanac data. For transmitting the acquisition-aiding signal a frequency channel adjacent to one of the frequency channels used for the carrier signals of the navigation signals is used so that there is only little interference with the adjacent frequency channel of the carrier signal. Due to the narrow frequency separation between the acquisition-aiding signal and the adjacent carrier signal the carrier signal and the acquisition-aiding signal can be received by the same antenna and receiver.
WULLEMS, C.; POZZOBON, O. and KUBIK, K.: Signal Authentication and Integrity Schemes for Next Generation Global Navigation Satellite Systems, European Navigation Conference GNSS, July 2005, Munich, Germany discloses methods for the authentication of satellites.
An overview over global positioning systems can be found in MISRA, P. and ENGE, P. Global Positioning System—Signals, Measurements, and Performance Ganga-Jamuna Press, 2nd ed., 2004.
Proceeding from this related art the present invention seeks to provide an improved method for transmitting position data from satellites to a navigation device.